Zinc-aluminum alloys with good machinability

ABSTRACT

Zinc-aluminum alloys having good machinability contain from 0.01 to 1 percent magnesium, from 0.01 to 3 percent bismuth, and up to 10 percent copper, in addition to zinc and aluminum in amounts to result in the alloy having a eutectoid transformation. Heat treatment of the alloys by homogenization at a temperature between the solidus and eutectoid temperatures and subsequent cooling to ambient temperature improves the corrosion resistance.

United States Patent [191 Gervais et al.

[ Mar. 19, 1974 ZINC-ALUMINUM ALLOYS WITH GOOD MACHINABILITY [75] Inventors: Edouard Gervais, Montreal,

Quebec; Pierre Chollet, Pierrefonds, Quebec, both of Canada [73] Assignee: Noranda Mines Limited, Toronto, Ontario, Canada [22] Filed: May 5, 1972 [2]] Appl. No.: 250,557

[30] Foreign Application Priority Data July 21, 1971 Canada 118722 [52] US. Cl 75/178 AM, 75/178 A [51] Int. Cl. C22c 17/00 [58] Field of Search 75/178 AM, 178 R, 178 A [56] References Cited UNITED STATES PATENTS 1.945.288 1/1934 Morel] 75/178 AM 3.676.115 7/1972 Hare et al 75/178 AM Primary Examiner-Hylznd Bizot Assistant ExaminerE. L. Weise [57] ABSTRACT improves the corrosion resistance.

5 Claims, No Drawings ZINC-ALUMINUM ALLOYS WITH GOOD MACHINABILITY This invention relates generally to zinc-aluminum alloys which contain, as essential alloying elements, bismuth and magnesium and which demonstrate good machinability and satisfactory corrosion resistance.

Many zinc-aluminum alloys are known for use in such applications as the production of wrought, die cast and gravity cast products and are known to be suitable for machining. However, alloys of the present invention have a better machinability over such known alloys. This is due to their improved chip forming characteristics.

The ease of machining is measured in terms of the surface finish, dimensional control, chip length and energy consumption and is known to depend upon such features as tool design, nature of lubricant, speed of machining, feed and amount of material removed at each revolution. It also depends on the alloy composition of the workpiece, its microstructure and its properties. The microstructure is influenced by such factors as the casting conditions, the conditions of hot working such as extrusion, the amount of cold working such as drawing and heat treatments.

While it has been suggested to improve the ease of machining of certain alloy systems, for example, zinccopper-titanium type alloys, by adding such alloying elements as lead and bismuth, these alloying elements have not been used with zinc-aluminum alloys. One reason for this is that, at the impurity level, the elements lead and bismuth have an adverse effect on the corrosion resistance of cast zinc-aluminum alloys (for example, the commercial die casting alloys).

A primary object of the present invention is to provide a zinc-aluminum alloy which has improved machinability and also possesses satisfactory corrosion resistance. It has now been found that zinc-aluminum alloys of improved machining characteristics and good resistance to corrosion are those which have a zincaluminum eutectoid transformation and contain up to 10 percent of copper, from 0.01 to 1 percent of magnesium and from 0.01 to 3 percent of bismuth, the balance, apart from incidental impurities, being zinc and aluminum, all percentages being by weight, and wherein the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth to form the intermetallic compound Bi Mg For a given screw machining set-up, tool design and lubrication, it has been found that bismuth in accordance with the amounts of the present invention, improves the chip forming characteristics of zincaluminum alloys without affecting the other machining characteristics of the part and at the same time, maintaining an acceptable level of corrosion resistance.

Especially, according to the present invention, a zincaluminum base alloy with improved machining characteristics and corrosion resistance comprises 18 percent to 30 percent aluminum, 0.01 percent to 1 percent magnesium, 0.01 percent to 3 percent bismuth, to about 5 percent copper, the balance being zinc except for incidental impurities and the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth to form the intermetallic compound Bi Mg This alloy would normally have a composition in which the percentage of magnesium equals 0.l times the percentage of bismuth plus an excess of magnesium.

Further, according to the present invention a method is provided for heat treating a zinc-aluminum base alloy which has a zinc-aluminum eutectoid transformation, to obtain improved corrosion resistance, wherein said alloy contains 0.01 percent to 3 percent bismuth, 0.01 percent to 1 percent magnesium, 0 to 10 percent copper such that the percentage of magnesium equals 0.175 times the percentage of bismuth plus an excess of magnesium, said method comprising homogenizing the alloy at a temperature below the solidus temperature and above the eutectoid temperature of said alloy and rapidly cooling the alloy to room temperature.

Therefore, according to the present invention, the relationship of the magnesium content to the bismuth is such that the percent magnesium equals 0.l75 times the bismuth content plus an excess of magnesium. The addition of magnesium and bismuth in such a relationship is necessary to provide sufficient magnesium to combine with substantially all the bismuth to form the intermetallic compound Bi Mg (The factor 0.175 is obtained by dividing three times the atomic weight of magnesium by twice the atomic weight of bismuth). It is thought that the formation of this intermetallic compound makes possible the combination of good machinability with acceptable corrosion resistance.

The tensile properties of the alloys are not significantly affected by the presence of the Bi Mg intermetallic compound. However, if the amount of magnesium in the alloy is in excess of the stoichiometric amount required to form this intermetallic compound it is found that the mechanical properties of the resulting alloy, particularly its tensile and creep strength, are improved.

More preferred amounts of the various alloying elements used in accordance with the invention are:

Aluminum: from about 22 to about 27 percent by weight Copper: from about 0.5 to about 1.5 percent by weight Magnesium: from about 0.05 to about 0.6 percent by weight Bismuth: from about 0.2 to about 1.5 percent by weight where the magnesium content is at least 0.175 times the bismuth content plus at least 0.02 percent magnesium, the balance being zinc and incidental impurities.

A particularly preferred range of bismuth within the above broadly indicated range is from 0.2 to 0.6 percent bismuth with the appropriate amount of magnesium.

Among examples of zinc-aluminum alloys which have been found to have a good balance of physical properties are the following alloys:

a. 25 percent by weight aluminum; 1 percent by weight copper; 0.125% by weight bismuth; 0.086 percent by weight magnesium and the balance zinc and incidental impurities;

b. 25 percent by weight aluminum; 1 percent by weight copper; 3 percent by weight bismuth; 0.6 percent by weight magnesium and the balance zinc and incidental impurities;

c. 25 percent by weight aluminum; 5 percent by weight copper; 0.5 percent by weight bismuth; 0.l4

percent by weight magnesium and the balance zinc and incidental impurities;

d. 20 percent by weight aluminum; 1 percent by weight copper; 0.5 percent by weight bismuth; 0.14 percent by weight magnesium and the balance zinc and incidental impurities; and

e. 30 percent by weight aluminum; 1 percent by weight copper; 0.5 percent by weight bismuth; 0.14 percent by weight magnesium and the balance zinc and incidental impurities.

The Table l given at the end of this disclosure shows the effect of alloy composition on the corrosion resistance and tensile properties of some alloys falling within the scope of the invention. The corrosion resistance has been determined by suspending 3/8-inch diameter rods for ten days in air saturated by steam at 95C., and then determining the amount of corrosion on the rod. The degree of corrosion is reported as the percentage of corrosion affecting the original diameter of the rod. This steam corrosion test originated in the die casting industry and is now widely used in the zinc industry even though it has not been standardized by the A.S.T.M. It is used mainly to simulate failure by inter-granular corrosion. Experience has shown that alloys whose dimensions and mechanical properties such as impact and tensile strength are not significantly altered by the test will not suffer intergranular attack in atmospheric service. The tensile tests were carried out at a crosshead velocity of 0.250 in./min. and the total elongation was calculated over a 2 inch gauge length.

Sample 1 in the as-extruded and cold drawn by 31 percent condition of Table 1 has a Mg/Bi ratio lower than 0.175/1 and was corroded 24 percent of the diameter. The amount of corrosion is, however, reduced to 1 1 percent when the magnesium content is more than 0.175 times the bismuth content as can be seen from Sample 2. A further reduction in the amount of corrosion to percent is experienced when the excess of magnesium is still larger as can be seen from Sample 3. Sample 4 demonstrates the improved corrosion resistance obtained when the bismuth level is reduced to 0.24 percent. Sample 4 has improved chip forming characteristics and is corroded 5 percent in the asextruded and drawn by 31 percent condition.

Table I also illustrates the effect of the MgBi ratio on the tensile properties. When the Mg/Bi ratio is lower than 0.175 (Sample 1) the tensile properties are relatively low indicating that most of the magnesium is associated with the bismuth and does not influence the matrix properties. If the Mg/Bi ratio is above 0.175/l as in Samples 2 to 8, the tensile strength of the alloy is improved thereby indicating that the excess magnesium strengthens the matrix. By comparing the aluminum content of Samples 5 and 6 with Sample 2, the usefulness of the Mg/Bi addition is demonstrated over a wide range of aluminum composition where the amount of bismuth and magnesium remain almost constant. Likewise by comparing the corrosion resistance of Samples 2, 7 and 8, the suitability of using from 0 to 4.2 percent copper is demonstrated.

The corrosion resistance of the alloys of this invention can further be improved by heat treatment. Table 11 also given at the end of the disclosure demonstrates the effect of various heat treatments on the corrosion resistance of an alloy which contains increasing amounts of bismuth with the appropriate level of magnesium. The amount of corrosion as a percentage of the specimen diameter is the measure of the amount of corrosion. Although increasing amounts of bismuth improve the chip forming characteristics of the alloy. increasing amounts of bismuth also decrease the corrosion resistance of the alloy as can be seen from the data in each column of Table ll.

Each of the heat treatments listed in Table 11 improved the corrosion resistance of the alloy in comparison to the corrosion resistance of the alloy in the asextruded and drawn by 31 percent condition.

Heat Treatment No. 1 of Table 11 involves furnace or slow cooling from 380C. which is a heat treatment described in US. Pat. Application No. 108,199 filed Jan. 20, 1971. Heat Treatment No. 2 of Table II involves furnace or slow cooling from 380C. which is interrupted by air cooling at 250C. This is a heat treatment described in our copending US application being filed of even date herewith.

Heat Treatment No. 3 of Table I1 involves rapid or air cooling from 380C. which is below the solidus temperature of the alloy. in the three heat treatments the alloys were homogenized for 15 minutes at the temperature prevailing before the cooling phase of the heat treatment.

In the case of Alloy No. 9 of Table 11 having 3 percent bismuth all three heat treatments improved the corrosion resistance to the same level (from 14 percent to 10 percent). in the case of Alloy No. 2 ofTable ll with 0.5 percent Bismuth Heat Treatment No. 3 resulted in the most improvement in corrosion resistance 1.5 percent from 1 1 percent) while Heat Treatment No. 2 resulted in the next most improvement in corrosion resistance (2.5 percent from 1 1 percent). In the case of Alloy No. 4 of Table II with 0.25 percent Bismuth Heat Treatments No. 2 and 3 demonstrated about the same degree of improvement in corrosion resistance (0.4 percent and 0.5 percent respectively from 5 percent). The improvement using Heat Treatment No. 2 was slightly greater than with Heat Treatment No. 3 (but the difference was not considered particularly significant).

However, in the Heat Treatment of Alloy No. 2 (0.5 percent Bi) Heat Treatment No. 3 produced significantly more corrosion resistance than Heat Treatment No. 2. Therefore, Heat Treatment No. 3 appeared to be the more preferred heat treatment for general application. Other experimental tests demonstrated that homogenizing for longer periods of time, such as 18 hours, further improved the corrosion resistance of samples treated by Heat Treatment No. 3.

In applying Heat Treatment No. 3 to alloys containing less than about 18 percent aluminum,- the solidus temperature becomes the eutectic temperature as is known from the zinc-aluminum phase diagram.

The alloys of the present invention clearly represent a useful advance in the art which should be of benefit to industry generally.

TABLET EFFECT OF ALLOYS COMPOSITION ON THE CORROSION RESISTANCE AND ON THE TENSILE PROPERTIES OF EXTRUDED AND DRAWN BY 31% ALLOYS Composition (Weight Magnesium in Ultimate excess of Mg/Bi Amount of Tensile Alloy weight ratio Corrosion Strength 7c 7: Reduc- Sample No. Al Cu Bi Mg of 0.175 Diameter lb/in Elongation tion Area Mg deficient alloy.

EFFECT OF HEAT TREATMENT ON AMOUNT OF CORROSION FOR ALLOYS CONTAINING VARIOUS LEVELS OF BISMUTH (0.175 X Bi 0.05% Mg).

Amount of Corrosion as of Specimen Diameter Under Various Metallurgical Condition Drawn 5% after Drawn 5% after furnace cooling Drawn 5% after air furnace cooling*** from 380C, intercooling**"'from Alloy As Extruded from 380C rupted by air-cool- 380C Sample and Drawn ing at 250C" Number by 31% (Heat Treatment No. 1) (Heat Treatment No. 2) (Heat Treatment No. 3)

9(3% Bi) [4 10 10 1O 2(0.5%Bi) ll 7 2.5 1.5 4(0.25%Bi) 5 4 0.4 0.5

' Heat treated according to Canadian Patent Application No. 030,768 Heat treated according to the co-pending Canadian Patent Application.

'" All alloy rods were homogenized minutes at temperature before heat treatment.

We claim:

1. A zinc-aluminum alloy having a zinc-aluminum eutectoid transformation said alloy containing about 18 percent to percent aluminum, 0.01 percent to 1 percent magnesium, about 0.01 percent to 3 percent bismuth, and 0 to about 10 percent copper, the balance, apart from incidental impurities, being zinc and aluminum, wherein the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth in the form of the intermetallic compound BIgMgg.

2. A zinc-aluminum alloy containing about 18 percent to 30 percent aluminum, about 0.01 percent to 1 percent magnesium, about 0.01 percent to 3 percent bismuth, 0 to about 5 percent copper, the balance being zinc except for incidental impurities wherein the ratio of magnesiuiri to bismuth is such aiofiitii iif ficient magnesium to combine with substantially all the bismuth in the form of the intermetallic compound 3. The alloy defined in claim 2 wherein the percentage by weight of magnesium equals 0.175 times the percentage of bismuth plus an excess of magnesium.

4. The alloy defined in claim 2 wherein the alloy contains 22 to 27 percent aluminum, 0.05 to 0.6 percent magnesium, 0.2 to 1.5 percent bismuth, 0.5 to 1.5 percent copper, the balance being zinc except for incidental impurities wherein the magnesium content is 0.175 times the bismuth content plus at least 0.02 percent magnesium.

5. The alloy defined in claim 4 wherein the bismuth content is between 0.2 percent and 0.6 percent. 

2. A zinc-aluminum alloy containing about 18 percent to 30 percent aluminum, about 0.01 percent to 1 percent magnesium, about 0.01 percent to 3 percent bismuth, 0 to about 5 percent copper, the balance being zinc except for incidental impurities wherein the ratio of magnesium to bismuth is such as to provide sufficient magnesium to combine with substantially all the bismuth in the form of the intermetallic compound Bi2Mg3.
 3. The alloy defined in claim 2 wherein the percentage by weight of magnesium equals 0.175 times the percentage of bismuth plus an excess of magnesium.
 4. The alloy defined in claim 2 wherein the alloy contains 22 to 27 percent aluminum, 0.05 to 0.6 percent magnesium, 0.2 to 1.5 percent bismuth, 0.5 to 1.5 percent copper, the balance being zinc except for incidental impurities wherein the magnesium content is 0.175 times the bismuth content plus at least 0.02 percent magnesium.
 5. The alloy defined in claim 4 wherein the bismuth content is between 0.2 percent and 0.6 percent. 